The universal amplitude ratios for ultrasonic attenuation above and below the critical point are determined for uniaxial magnets, the gas liquid critical point, binary mixtures and related universality classes. For uniaxial ferromagnets also the scaling functions are computed. The theory is based on time dependent Ginzburg-Landau models and the ɛ=4−d-expansion. Depending on the universality class the universal amplitude ratio assumes values in the range 1/15 to 15. In systems with continuous internal symmetry Goldstone mode singularities are found.